Field of the Invention
The present invention relates to a DC-AC converter comprising a bridge of electronic switches having a load in one diagonal and a constant voltage source in the other diagonal.
Description of the Prior Art
A DC-AC converter of the type generally set forth above is disclosed in the German published application No. 28 34 561, fully incorporated herein by this reference. X-ray technology is one area of use for such a DC-AC converter. Here, the DC-AC converter can serve the purpose of feeding the high voltage transformer of an X-ray generator. In this case, the feed of the high voltage transformer occurs with a frequency lying specifically above the commercial power line frequency so that the high voltage transformer can be designed considerably lighter in weight and smaller in comparison to X-ray generators supplied directly from the commercial power lines.
The reliable quenching of the bridge short circuit by a short circuit quenching device is of considerable significance for the reliable operation of a DC-AC converter of the aforementioned type. Bridge short circuits can never be entirely excluded and can be caused, for example, by malfunctions of the X-ray tube. The aforementioned German application therefore discloses a short circuit quenching device which prevents a response of the main fusing given a sporadic misdrive of the DC-AC converter switches (thyristors) which would mean a case of service with all of its disadvantages and costs.
A quenching device of the aforementioned type, however, only operates reliably when the distribution of the short circuit current is approximately equal in the individual DC-AC converter branches. Given more than one DC-AC converter, however, i.e. given the parallel feed of the load by a plurality of DC-AC converters, for example an X-ray tube by two or more DC-AC converters connected in parallel, this uniform current distribution is difficult to realize, in particular because of the component tolerances and the parasitic inductances. Given the use of one AC-DC converter, there can already be problems with respect to the uniform distribution of the short circuit current when the undesired wiring inductance is too high.